Spain is famous for its weather, Benidorm, Spanish food, it's football team, Barcelona, and basically being a rather sunny place overall â none of these descriptions are incorrect, and in fact they are all entirely true!
As the temperature of the air increases its density decreases. Therefore the thrust produced by the engine decreases. As the air pressure increases, its density increases, causing thrust produced by the engine to increase. Altitude has a double effect on thrust. As the altitude increases, the air becomes colder and denser, up to the beginning of stratosphere. This causes the thrust to increase.
where T is the thrust generated (force), is the rate of change of mass with respect to time (mass flow rate of exhaust), and v is the speed of the exhaust gases measured relative to the rocket. For vertical launch of a rocket the initial thrust at liftoff must be more than the weight.